Abstract: Mars is a very cold planet, drier than any desert on Earth, and it has a carbon dioxide atmosphere. Yet, clouds composed of water or CO2 ice crystals form in its atmosphere from near the surface up to 100 km altitude. These clouds form on ice nuclei provided by the omnipresent mineral dust, lifted by winds from the surface, or provided by meteors ablated in the atmosphere. Despite their thinness, the water ice clouds have a significant longwave radiative effect, and the polar winter CO2 clouds contribute to the formation of the polar caps and the annual variation of the surface pressure.

Venus, when seen in visible wavelengths, looks like a billiard ball. This is due to the thick, highly reflective layer of clouds that covers the whole planet and that is composed of sulphuric acid droplets. How the clouds form, on which condensation nuclei, and how the observed multimodal size distribution is maintained, are still mysteries. The clouds have a huge effect on the climate by reflecting and absorbing most of the sunlight, allowing less than 5% of it to reach the planet’s surface, where, nevertheless, the temperature reaches a soaring +460 degrees Celsius.

In this seminar, I will give an overview of the cloud systems of the two planets. Then I will discuss in more detail some examples on the formation of these clouds and recent advances on the topics.

I am a Finnish research scientist who has been working in France for ten years. Though a meteorologist by formation, I turned to stars very early, and ended up with a Master’s thesis and a Ph.D. thesis (2007) in the University of Helsinki, Finland, focused on modelling the atmosphere of Mars. I left Finland after my PhD for a post-doctoral fellowship in France, and concentrated on analysing cloud observations from the Mars Express satellite. I was hired as a permanent research scientist of the CNRS on my second year of post-doc, which has allowed me to broaden my research to the clouds on Venus and to deepen our understanding on the formation processes of Martian mesospheric clouds through developing models of cloud microphysics and analysing satellite observations in a large international collaboration network.